1. Field of the Invention
The present invention relates to an apparatus for controlling the speed of an elevator.
2. Description of the Related Art
FIG. 6 is a view showing the structure of the elevator control apparatus disclosed in Japanese Patent Laid-Open No. 60-16184. A thyristor convertor 2 is connected to a three-phase alternating Current power source 1. A transistor invertor 4 is connected to the thyristor convertor 2 through a smoothing capacitor 3. The thyristor convertor 2, the smoothing capacitor 3, and the transistor invertor 4 constitute an electric power convertor.
An induction motor 5 for driving an elevator is connected to the transistor invertor 4. A speed detector 6 for detecting the rotational speed of the induction motor 5 is connected to the induction motor 5. An electric current detector 7 is disposed between the transistor invertor 4 and the induction motor 5. A regulator 8 for controlling the electric power convertor is connected to the speed detector 6 and the electric current detector 7. The regulator 8 has an interface (I/F), a ROM, a RAM, and a CPU, and performs a comparison operation between a speed command .omega.r* from a speed command device (not shown) and a detection signal .omega.r from the speed detector 6 affixed to the induction motor 5. The regulator 8 supplies the transistor invertor 4 with a PWM signal based on the output of the electric current detector 7.
Furthermore, a sheave 9 is coupled to the induction motor 5. A hoisting rope 11 is fitted over the sheave 9 and a deflector sheave 10. One end of the hoisting rope 11 is joined to a car 15, whereas the other end is joined to a counterweight 12. A tension pulley 13 is disposed under the above components. One end of a compensating rope 14 wound around the tension pulley 13 is joined to the car 15, the other end being joined to the counterweight 12. Also, an electromagnetic brake 16 is disposed above the external circumference of the sheave 9.
FIG. 7 is a block diagram showing the regulator 8 of FIG. 6. The regulator 8 is composed of a speed control amplifier 21, a differentiator 22, dividers 23 and 37, coefficient multipliers 24 to 28, a computing device 29 for a DC component vector, and adders 30 and 31. The regulator 8 is further composed of a vector oscillator 32, a vector multiplier 34, a vector three-phase convertor 35, and an operational amplifier 36.
The regulator 8 takes in the speed command .omega.r* and a secondary magnetic flux command .phi..sub.2 *, as well as the detection signal .omega.r from the speed detector 6. The detection signal .omega.r and the speed command .omega.r* are input to the speed control amplifier 21. The value obtained by amplifying the deviation between the detection signal .omega.r and the speed command .omega.r*, is regarded as a torque command T.sub.M *. The divider 23 divides the torque command T.sub.M * by the secondary magnetic flux command .phi..sub.2 * to determine a secondary q axis current command i.sub.2.sbsb.q *. The coefficient multiplier 24 multiplies the secondary q axis current command i.sub.2.sbsb.q * by L.sub.2 /M to determine a torque component current command i.sub.1.sbsb.q *. L.sub.2 is the self-inductance of a secondary rotor winding, and M is the mutual inductance of a primary stator winding and a secondary winding.
After the differentiator 22 has differentiated the secondary magnetic flux command .phi..sub.2 *, the coefficient multipliers 26 and 25 multiply it by 1/R.sub.2 and L.sub.2 /M, respectively. The secondary magnetic flux command .phi..sub.2 * is then input to the adder 30 in the form of an electric current. This electric current is used for forcing secondary magnetic flux proportional to a time variation rate. R.sub.2 is secondary winding resistance. The coefficient multiplier 27 also multiplies the secondary magnetic flux command .phi..sub.2 * by 1/M. The secondary magnetic flux command .phi..sub.2 * is input to the adder 30 as an exciting current to obtain secondary magnetic flux.
On the other hand, the coefficient multiplier 28 multiplies the secondary q axis current command i.sub.2.sbsb.q * by R.sub.2. The divider 37 divides the secondary q axis current command i.sub.2.sbsb.q * by the secondary magnetic flux command .phi..sub.2 *. The secondary q axis current command i.sub.2.sbsb.q * is then input to the adder 31 in the form of a slip frequency command .omega.s*. The adder 31 adds the slip frequency command .omega.s* to the detection signal .omega.r in order to determine a speed command .omega.o* of the secondary magnetic flux. The speed command .omega.o* is in turn input to the vector oscillator 32 which integrates it to calculate a phase .sub.e j.theta.o* of a primary current command.
A computing device 29 for a magnetic flux vector, on the other hand, calculates a root-mean-sequare value i.sub.1 (.theta.o)* of the primary current command. This calculation is based on a torque component current command i.sup.1q * and a magnetic flux component current command i.sup.1d *. The phase and the root-mean-sequare value thus obtained are multiplied by the vector multiplier 34 and the vector three-phase convertor 35 to generate three-phase primary current commands iu*, iv*, and iw*. These three-phase primary current commands iu*, iv*, and iw* are output to the transistor invertor 4 via a PWM modulator 36.
The conventional elevator speed control apparatus is constructed as above. When an elevator stops, a main contactor (not shown) is opened, and the exciting current of the induction motor 5 is intercepted. However, soon after a magnetic flux current component has been intercepted, the secondary magnetic flux of the induction motor 5 does not become extinguished, but instead, attenuates at the time determined by a secondary time constant. For this reason, when the elevator is started again immediately after the main contactor has been opened, the secondary magnetic flux of the induction motor 5 does not attenuate yet to the fullest extent. Thus when the magnetic current component is forced to raise the secondary magnetic flux, the secondary magnetic flux is excited excessively. This results in a problem in that a start shock or balance variation occurs.